1. Field
Various features relate to wireless communication devices and systems, and more particularly to methods and apparatuses for generating preambles for television white space transmissions.
2. Background
Access terminals, such as laptop computers, personal digital assistant devices, mobile or cellular phones, personal media players, or other devices with a processor, that communicate through wireless signals are becoming increasingly popular and are used more frequently. Many access terminals are adapted to communicate via Wireless Local Area Networks (WLAN), and several emerging wireless communications standards have been developed and are being developed for facilitating such WLANs.
The Institute of Electronics and Electrical Engineers (IEEE) has developed various WLAN standards (or revisions) under the umbrella of its IEEE 802.11 standard. IEEE 802.11 denotes a set of WLAN air interface standards developed by the IEEE 802.11 committee for short-range communications (e.g., tens of meters to a few hundred meters). Such WLAN networks are often referred to as “WiFi” or “wi-fi” networks and allow electronic devices to communication wirelessly (e.g., exchange data, access other networks, etc.). Common wi-fi frequencies are include the 2.4 GHz and 5.6 GHz bands.
However, the available frequency spectrum for such wireless networks is quite limited and there is an ever increasing need for bandwidth demanded by wireless communications systems. As such, reuse of, and/or co-existence with, existing frequency spectrum allocations is desirable. One relatively recent development corresponds to implementing a WLAN using “white space” (also referred to as “whitespace”). White space may refer to radio frequencies allocated to a broadcasting service but not used locally (e.g., within short range). In the U.S., the Federal Communication Commission (FCC) Report and Order (R&O) recently opened up an opportunity of unlicensed usage of white space. Use of white space for implementing a WLAN according to the IEEE 802.11 standard is being developed under an amendment to the IEEE 802.11 referred to as IEEE 802.11af. “White-fi” is a term being used to describe the use of a Wi-Fi technology within the TV unused spectrum, or TV white space (TVWS). By using the TVWS with frequencies below 1 GHz, IEEE 802.11af may offer greater propagation distances to be achieved, in addition to the increased bandwidth made available by the locally unused frequencies in the TV spectrum.
In the U.S. each television channel has a total bandwidth of 6 MHz. However, due to concerns of spectral leakage into adjacent channels, signal providers typically limit the signal carrier bandwidth for each channel from 4 MHz to 5 MHz in order to provide 500 kHz to 1 MHz guard bands on each side of the channel. In other countries the total bandwidth for each television channel may be different (e.g., 8 MHz for most European countries).
TVWS devices can operate in outdoor environments, making them vulnerable to high delay spreads. This entails a much higher cyclic prefix (CP) in most other OFDM-based transmissions in order to provide sufficient protection against delay spread effects. As one example, the 802.11af signal may be a 5 MHz signal that is generated by down-clocking an 802.11ac signal. For example, a 20 MHz, 40 MHz, 80 MHz, or 160 MHz bandwidth 802.11ac signal may be down-clocked to a 5 MHz bandwidth signal using an appropriate down-clocking factor. Down-clocking the signal causes the cyclic prefix duration, preamble duration, and symbol duration to increase in time. The increase in time is directly proportional to the down-clocking factor applied. For example, a down-clocking factor of sixteen (16) used to down-clock an 80 MHz bandwidth signal to a 5 MHz signal increases the cyclic prefix duration by sixteen (16) times, whereas down-clocking a 20 MHz signal to 5 MHz only increases the cyclic prefix duration by four (4) times.
In some cases generating a 5 MHz TVWS signal by down-clocking a 20 MHz 802.11ac signal may result in a cycle prefix duration that is too short, which may cause the resulting signal to be vulnerable to the high delay spreads. By contrast, down-clocking a larger bandwidth (e.g., 40 MHz, 80 MHz, etc.) 802.11ac signal to generate a 5 MHz TVWS signal may result in a preamble duration that is too long. That is, the preamble duration may occupy an unacceptably large portion of the total time allotted to the WLAN device for a given TVWS transmission (i.e., the preamble overhead of the Physical Layer Convergence Protocol Packet Data Unit (PPDU) may be too great).
Accordingly, there is a need for a TVWS preamble (e.g., TGaf) that features a sufficient cyclic prefix duration to combat high delay spread, low preamble duration overhead, and the ability to re-use the same architecture as other IEEE 802.11 standards, such as 802.11ac.